"This man is a crucial witness in the investigation and would encourage him and anyone else with information relating to this incident to make contact in confidence as soon as possible."

Additional police patrols are being mounted across east London to reassure the public.

Two men have been arrested in connection with the investigation.

A 22-year-old man was arrested in Acton, west London on Sunday, and a 19-year-old man was arrested on Wednesday after going to an east London police station.

The pair were arrested on suspicion of causing grievous bodily harm and public order offences and were bailed to return to an east London police station in February and March pending further inquiries.

A police spokesman said: "The Metropolitan Police Service takes these incidents very seriously and is pursuing various lines of inquiry with a view to identifying and prosecuting the individuals concerned."